Ringtone goes from high to mute to high again. How to fix?

I've tried changing the ringtone but nothing works.
When someone calls me my ringtone starts at the volume I put it on and a second later it just shuts off and then it starts again a few seconds later and it does that all over again. Has anyone experienced this before and if yes how did you fix it?
Thanks in advance and sorry for my bad English.